FIG. 1A illustrates a wireless communication system 130, which includes a first transceiver 100 and a second transceiver 102. The first transceiver 100 may be designated a “transmitter” because it first transmits signals to the second transceiver 102, which may be designated a “receiver.” Both transmitter 100 and receiver 102 may transmit and receive wireless signals, as shown by the transmit portions 101A, 101B and receive portions 103A, 103B.
Orthogonal frequency division modulation (OFDM) is a modulation technique for communications which splits a data stream into multiple radio frequency channels, which are each sent over a “subcarrier” frequency.
In an OFDM baseband system, such as an IEEE 802.11a/g system, a transmitter 100 transmits two identical training sequences known as “preambles” to a receiver 102. The transmitter 100 sends these training sequences over two OFDM symbol durations. The transmitter 100 may send training subcarriers (also called “carriers” or “tones”) for the training sequences that correspond to all data subcarriers to the receiver 102.
FIG. 1B illustrates an example of a data burst/packet 120 transmitted by the transmitter 100 to the receiver 102 in FIG. 1A. In IEEE 802.11, the packet 120 contains a preamble 121A,B in a training mode and a plurality of data symbols 122A-122C in a data mode. In IEEE 802.16a, the packet contains a preamble 124 in a training mode and a plurality of data symbols 126A-C in a data mode.
FIG. 1C illustrates a set of subcarriers for a preamble “P” 204 at the transmitter 100 in FIG. 1A. FIG. 1C shows how the subcarriers are transformed and transmitted to the receiver 102. A subcarrier “i” in the frequency domain contains a training symbol. The transmitter 100 performs an inverse fast Fourier transform (IFFT) to transform the subcarriers to one time-domain OFDM symbol. One OFDM symbol contains multiple training symbols carried over the sub-carriers. The transmitter 100 transmits the OFDM symbol across a channel. The receiver 102 receives the OFDM symbol and performs an FFT operation. The receiver 102 estimates a frequency response Hi of the subcarrier i.
The receiver 102 typically uses two identical received training sequences/preambles to compute their correlation and obtain time and frequency information for frequency synchronization and channel estimation. The receiver 102 may compute channel estimation using a Least Square (LS) estimator. An estimated channel frequency response Hk corresponding to a subcarrier k (k is an index for subcarriers) may be expressed as:
                                                        H              _                        k                    =                                                                                          P                    _                                    k                                                  P                  k                                            ⁢                                                          ⁢              for              ⁢                                                          ⁢              k                        =            0                          ,                  1          ⁢                                          ⁢          …                ⁢                                  ,                                  ⁢                  N          -          1                ,                                  ⁢                              and            ⁢                                                  ⁢                          P              k                                ≠          0                ,                            (        1        )            
where N denotes the total number of subcarriers of an OFDM symbol, Pk is the received symbol of the kth subcarrier, and Pk is the corresponding training symbol for that kth subcarrier, which forms a part of a preamble. Equation (1) indicates that a channel estimate may be determined for each subcarrier k.
A preamble represents information about packets that follow the preamble. As such, preambles are overheads of a packet to be transmitted. There may be multiple preambles. Preambles reduce the throughput of a communication system. For a diversity system, such as a multiple-in-multiple-out (MIMO) system with multiple transmit and receive antennas 104, 106, channels between all pairs of the transmit and receive antennas 104, 106 should be estimated. Extra training overheads may be required.
It may be advantageous to minimize the number of preambles required to be transmitted while still obtaining satisfactory system performance. To increase system throughput and reduce the overhead of transmitting two identical training sequences over two OFDM symbol durations per packet, the IEEE 802.16a OFDM system proposes transmitting two identical training sequences using one preamble over one OFDM symbol duration. The receiver 102 uses the two identical sequences for correlation to obtain frequency information. To obtain two identical training sequences in the time domain over just one symbol duration, every other subcarrier in the frequency domain has to be set to zero, which is a known Fourier transform property.